1. Field of the Invention
This invention relates to weighing systems having a plurality of electronic load cells, and in particular to such weighing systems in which the load cells are of a center-loaded type which is subjected to side loading.
2. Description of the Prior Art
Weighing systems are commonly used with material handling apparatus, such as ladle crane hangers and scrap chargers, which convey loads which are unevenly distributed over the material handling surface.
A weighing system in conjunction with a ladle crane hanger permits the operator to know how much molten metal is initially in the ladle and how much remains after each pour. In practice, the load cells in the weighing system sense the weight loss of the molten metal leaving the ladle when pouring the ingots and, through appropriate electronics, display this weight loss as the weight of the ingot which has been poured. Knowing the weight of the molten metal is very important in pouring ingots to an accurate weight. Ingot molds erode internally with use over a period of time, and the practice of pouring an ingot to a mark on the mold does not produce an accurate ingot weight. By knowing the weight of the molten metal being poured, the ingot can be poured to an exact ordered weight rather than to an estimated weight. This information also enables the operator to determine when there is not enough molten metal remaining for another ingot, and thereby presents the waste of an ingot due to an insufficient amount of metal.
In the case of ladles containing scrap metal knowing the exact amount of scrap metal being charged into the steel-producing furnace improves the control of the recipe and, therefore, the quality of the steel produced. This is also the case when charging the furnace with molten iron.
Despite the advantages of using a weighing system in a material handling apparatus, serious disadvantages exist which may render the weight measurements inaccurate. Loads handled by the ladle crane hangers and scrap chargers do not have a uniform distribution of weight over the material handling surface. In many prior weighing systems, this uneven load distribution rendered the weighing systems inaccurate or unusable for many purposes. The uneven load distribution often causes the limits of a particular cell or cells to be encountered. When this occurs, the reading transmitted from that cell is highly inaccurate and non-repeatable. Inaccurate readings of the weight of molten steel in a ladle can result in scrap ingots due to an insufficient amount of steel. The economic consequences of such errors can be serious.
One attempt to overcome these problems is illustrated in U.S. Pat. No. 3,899,034, issued to Polen et al. In this patent, a weighing system is disclosed which was comprised of two fixed load cells at one side of the sheave block and two movable load cells at the opposite side of the sheave block. Each of the load cells was a conventional columnar compression load cell which had to be loaded vertically and could not withstand side loading. The movable cells were intended to adjust the load distribution to avoid side loading. However, the movable load cells produced an instability which usually required devices such as check rods to protect the load cells and to assure that no contact existed between the ladle hook support structure and the sheave basket. Any contact between the sheave basket and the ladle hook support structure other than a vertical force or load applied to the load cells would absorb load and manifest itself as an inaccuracy in the sensed weight or cause non-repeatable load variation when dynamic conditions were encountered, such as the moving of the crane, the placing of the ladle hanger on the ground for maintenance, or the raising of the ladle hanger. While the load-absorbing characteristics of the check rods were predictable, the check rods could still reduce the accuracy of the weight sensing devices.
One approach to eliminating the disadvantages of these check rods is disclosed in patent application Ser. No. 314,361, filed Oct. 23, 1981, now U.S. Pat. No. 4,413,691, of which this application is a continuation-in-part. In that application, three fixed bending beam load cells were arranged in a stable three-point configuration which eliminated the need for check rods, and therefore, improved the accuracy of the weight distribution. However, since a point contact was used at each of the three points, a certain amount of lateral movement of the ladle hook support structure was possible. This movement could cause contact with the load cell transducer by the support collar on which the ladle hook support structure was supported. The result of this contact would be load absorption which subsequently would manifest itself as an inaccuracy in the sensed weight, and hence an inaccurate measured weight displayed to the operator.
In order to stop or inhibit this lateral sliding, application Ser. No. 314,361 disclosed the use of flexure plates mounted on the bottom of the sheave basket on each side which were attached to the ladle hook support structure. The flexure plates maintained a fixed distance between the ladle hook support structure supported on the load cell support collar and the sheave basket upon which load cell transducers were mounted. Another method to hold the relative position between the sheave basket and the ladle hook support structure was the use of check rods. Flexure plates and check rods were installed so that they are in a bending mode vertically and in a compression or tension mode horizontally. These devices always introduced inaccuracies to the system. Even when the load absorbing characteristics of the flexure plates or the check rods were predictable, these devices still reduced the accuracy and repeatability of the readings. A further disadvantage of check rods and flexure plates is that they invariably became loose and were generally unreliable. Due to the construction of the sheave block assembly, it is very difficult, or almost impossible, to gain access to the check rods to assure that they were tight. Due to the shock and vibration encountered in the environment in which the ladle hanger operated, the check rods and flexure plates invariably became loose and caused great sensed weight variations and could result in potential damage to the load cell transducer.